The invention relates to a method and arrangement for eliminating or reducing the sound reproduction effect in head-phones due to the turning of one's head. The invention relates more particularly to specific head-phones with sound-reproducing characteristics normally associated only with large speakers spaced a distance from the listener. Such head-phones simulate the spaciousness of the sound reproduced by a larger speaker, and particularly that produced by a plurality of speakers spaced apart from one another, in a head-phone set.
Head-phones are used nowadays in ever greater numbers, such as for listening to radio broadcasts, phonograph records, and tape recordings. Furthermore head-phones are used for technical audio purposes, such as for monitoring purposes during recording sessions, live broadcasting and so-called play-back techniques.
One of the chief reasons for the increasing popularity of head-phones for home use is that they permit the listener to hear live broadcast or recorded material without disturbing other persons not wishing to listen, and likewise prevents the listener from being distracted by other sources of sound in the room in which he is present. However, there are significant disadvantages associated with the use of head-phones, as opposed to ordinary loud speakers. Head-phones, even those of high quality, exhibit sound reproducing characteristics which are very different from those of loudspeakers. These different sound reproducing characteristics include not only difference in frequency response, but equally important differences in the sense of acoustical spaciousness and direction of sound experienced by the listener.
When a head-phone set is plugged into the same electrical outputs into which are plugged the inputs of a loudspeaker system, very marked differences are observed in the acoustical effects received by the listener from using the earphones of the headset, instead of the loudspeaker. Aside from minor differences in frequency response, there are differences of a psychological nature, relating to the spatial characteristics of the received sound. For example, the listener often perceives that the orchestra is located within the head of the listener or at a distance from the listener's head on the order of magnitude of the distance between the listener's ears, rather than at a remote location from the listener. This is particularly true when the listener is listening to loud music, which is frequently the case when listening to high-quality stereophonic equipment.
It has been extremely difficult to deal in a systematic and scientific manner with these psychological phenomena. The causes of these phenomena have always been assumed to include such factors as unavoidable differences in the sound-producing characteristics of the head-phone sets, the exact positioning of the earpieces of the head-phones, with respect to the listener's ears, the pressure with which the earpieces press against the listener's ears, the sound transmissivity of the skull bone of the particular listener, the effect of the listener of moving his head while listening and other such physiological and psychological factors. In listening to head-phones, the electro-acoustical transduction phenomena does not include the factor of substantial transmission distance, sound dampening, sound distribution within the room between the listener and speaker, and the combination of sound before the sound reaches the listener's ear; instead, the total electro-acoustical transduction depends directly on the tranducer characteristics of the earphones in the head-set rather than the external environment of the listener. There have been a number of methods directed at eliminating both the spatial and spectral distortion associated with the use of head-phone set, i.e., as specifically compared to the spatial and spectral phenomena associated with high-quality loudspeakers employed to listen to the same material.
German Offenlegungsschrift, No. 1,927,401 discloses one such attempt to deal with the problem. According to the approach in question, experiments were conducted on an artificially constructed human head provided with two microphones in the region of the ears of the head. The acoustical characteristics of an actual human head were simulated to the greatest extent possible, and measurements were taken of the sound reception in the ear canals' locations of such head. As a result of the measurements taken, recording engineers were able to modify their recording technique in such a manner as to produce recordings or broadcasts which, when listened to with earphones, will have the desired improved spatial and spectral characteristics. This approach is, however, of little practical value. It would necessitate the establishment of an entirely new category of recording equipment and broadcasting channels which would be used with earphone reception specifically in mind. This is evidently undesirable because it would entail the manufacture of duplicate records and tapes, and the transmissions of broadcasts falling into one category or another, with the listener being compelled to listen to the selected one, or else settling for a considerable amount of distortion.
Another method for imparting to head-phones the sound-reproducing characteristics of loudspeakers is set forth in U.S. Pat. application Ser. No. 395,371 now U.S. Pat. No. 3,920,904. This method entails furnishing an electrical network having a network transfer function corresponding to a predetermined function of both the desired transfer function and the earphone transfer function.